WebPerf Snippets is a collection of JavaScript snippets that run in browser developer tools to measure and debug website performance. It helps developers investigate Core Web Vitals, loading, interaction responsiveness, images, video, scripts, and other performance issues. Its catalogue skills, setting, and instruction support agent-assisted use of the snippets.
Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add nucliweb/webperf-snippets --skill webperf-mediagit clone --depth 1 https://github.com/nucliweb/webperf-snippetsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-media)<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-media"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-media/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/nucliweb/webperf-snippets/webperf-media"><img src="https://agentmods.dev/badge/skills/nucliweb/webperf-snippets/webperf-media.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector warn
SkillSpector: 1 finding, up to medium
These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →
- medium Excessive Agency · line 90 Skill enables autonomous high-impact decisions without human-in-the-loop verification. Critical operations (destructive commands, financial transactions, data deletion) should require explicit user confirmation.Fix: Add human-in-the-loop confirmation for destructive, irreversible, or high-impact operations. Never auto-execute commands that modify files, send data, or alter system state.
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00155 | $0.02859 |
| Opus 5.5 | $0.00062 | $0.01144 |
| Sonnet 5.5 | $0.00031 | $0.00572 |
| Haiku 4.5 | $0.00015 | $0.00286 |
Grade A, and why
webperf-media scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 248 lines — stays where its author put it; the contents beside it link to each section on GitHub.
WebPerf: Media Performance
JavaScript snippets for measuring web performance in Chrome DevTools.
Running a script
- Run the script with
mcp__chrome-devtools__evaluate_scriptand read the object it returns. The scripts have no console output, so the returned object is the result:script,statusand the metric data (value,rating,details,items,issues), as described inreferences/schema.md. - When
statusis"tracking", the script keeps observing the page. After the interactions, call the function named ingetDataFnwith anothermcp__chrome-devtools__evaluate_scriptcall, for examplegetINP(), and read the object it returns. - When
statusis"error"or"unsupported", readerror: the page may not have loaded yet, or the browser lacks the API.
Without Chrome DevTools MCP, run the same snippet with the webperf-snippets CLI, which loads the page in headless Chromium and prints the same result objects: npx webperf-snippets <url> --snippet <Name> --json, where <Name> is the script name without .js. It needs Playwright and Chromium (npm install --save-dev webperf-snippets playwright, then npx playwright install chromium), and tracking snippets take --interact-script <file> to drive the interactions.
Scripts
scripts/Image-Element-Audit.js— Image element auditscripts/Oversized-Images.js— Oversized imagesscripts/SVG-Embedded-Bitmap-Analysis.js— SVG embedded bitmap analysisscripts/Video-Element-Audit.js— Video element audit
Common Workflows
Complete Media Audit
When the user asks for media optimization or "audit images and videos":
- Image-Element-Audit.js - Analyze all images (format, lazy loading, sizing, fetchpriority)
- Video-Element-Audit.js - Analyze all videos (poster, preload, formats, autoplay)
- SVG-Embedded-Bitmap-Analysis.js - Detect inefficient bitmap images embedded in SVGs
Image Optimization Workflow
When the user asks "optimize images" or "check image performance":
What ships with it
6 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago Changed · -1 lines 516694cdcdf7
- 5d ago Changed · +8 lines fa9eb06c6fbf
- 6d ago Changed 8d2d7d2f61b6
- 21d ago Changed · +23 lines be04debbb046
- 22d ago Changed d25a2a3176f1
- 1mo ago First seen · 218 lines · 155 tokens per session scan A 40a879e2b617
webperf-media is a skill published in the GitHub repository nucliweb/webperf-snippets (1,454 stars, last pushed today), licensed MIT. It adds 155 tokens to every session and 2,859 once invoked, about $0.0006 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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